1. Field of the Invention:
The invention relates generally to a telecommunication system, and more particularly to a computer controlled system for controlling electronic signs using radiopaging techniques.
2. Description of the Related Art:
A method of cost-efficient, centralized control of electronic displays that does not require a physical communication link between the displays and a central computer is advantageous because the method allows the placement of electronic information displays in areas where such a communication link is prohibitively costly and sometimes impossible. Several methods of centralized control of electronic signs exist. One method uses a standard telephone line to connect each electronic display to the Public Switched Telephone Network (PSTN). Another telephone line connects a central control computer to the PSTN. The control computer and the electronic signs each use a telephone MODEM to send and receive data. In this configuration, the central computer can place a telephone call to each sign or a group of signs to upload programming information over the PSTN using an end-to-end protocol such as XMODEM or X.25. This method works well for a small number of signs. Large organizations of signs, however, require many dedicated telephone lines. The use of telephone lines with a large organization of signs is disadvantageous because communication cost increases linearly with the number of displays. Additionally, the telephone lines are inconvenient and sometimes impossible to install in some locations. Further, the use of telephone lines with electronic signs limits the mobility of the displays. Placing displays interconnected in such a fashion onto public transportation vehicles, for example, is impossible.
The mobility limitation can be eliminated by substituting a cellular or mobile telephone link for the conventional telephone line, allowing connection between a central controlling computer and electronic displays without a physical connection to each sign. This approach is nonetheless disadvantageous because a call over the mobile telephone link must still be completed for each sign receiving display data. Thus, line cost continues to increase linearly with the number of signs in the network.
A number of approaches for providing electronic signs with display data which address both sign mobility and communication cost exist. A first method places an FM receiver into each sign on the network and modulates the data intended for each sign onto the subcarrier of a commercial broadcasting station. This method offers fixed costs and mobility, but presents a number of disadvantages. First, the subcarrier signal carries only a fraction of the broadcast power of the commercial stations. Second, modulation of the data in this way reduces the power of the signal and degrades the overall signal quality. Third, FM broadcast paging companies generally do not perform simultaneous broadcasts of radiopage signals to a number of page receivers.
A second method uses existing radiopaging technology by placing an alphanumeric pager (i.e., pager receiver) within each electronic sign. Messages intended for the signs are then transmitted as alphanumeric radiopages that are received by pagers within the signs. The alphanumeric pager outputs the data onto its printer port, which is connected to a compatible port within the sign itself. The data is then received and stored in the sign's memory. This approach has the advantage that it does not interfere with the primary operation of the paging company, but it imposes the disadvantage of limiting messages to the length of an alphanumeric page and restricting characters outside the ASCII 7-bit printable character set. These restrictions preclude the transmission of large bit mapped graphic images.
Other methods for providing display data to electronic signs involve creating a format which either allows longer pages or allows messages to be spread over multiple pages. A method that allows messages of arbitrarily long duration is undesirable because long messages cause delay in transmitting other pages and thereby adversely effect the overall response time of the paging system. Spreading a long message over multiple pages is difficult because paging systems use time slot address selection to allow pagers to efficiently use their batteries. Paging systems often re-sort pages from their original chronological order before transmission to minimize transmission of idle code words. Thus, recovery of re-sorted data is rendered more difficult at the paging receiver. Some paging formats exist for creating long pages from several short pages; however, these formats do not allow for the reordering of the pages. Further, these paging formats would require paging companies to replace existing hardware and undergo extensive software upgrades.